Despite abundant conjecture to the contrary, it has been established for the first time that amorphous silicon (a-Si) technology is more than adequate to meet the pixel current drive requirements of an active matrix organic light-emitting diode (AMOLED) display. Prevailing wisdom, based almost exclusively on the industry's familiarity with AMLCD a-Si backplanes, suggests that even if current drive requirements can be met using a-Si thin film transistor (TFT), the well-known threshold instability of such devices precludes their use in a voltage-programmed active matrix design, since any loss of current drive in the OLED element results directly in a loss of luminance, whereas in an AMLCD, loss of TFT current results only in an increase in the pixel capacitance charging time (on the order of μs) rather than in a change in the final voltage, hence luminance levels may remain unchanged for voltage shifts as large as 10V for AMLCDs. It should be pointed out, however, that the range of voltages and the drive regime of the current drive TFT in an AMOLED display are, and in fact must be, dramatically different. Refer to a FIG. 1A showing typical one TFT AMLCD pixel circuit schematic, and an illustrative FIG. 1B showing a two TFT AMOLED pixel. Consider the TFT in FIG. 1A which serves only as a switch in charging the parallel combination of pixel LC capacitance (CLC) plus the storage capacitance (Cs). This switch has a duty cycle of 100/#R where #R is the total number of rows in the display, which typically ranges from 640 to 1200 for the most common designs available today with pixel content of VGA to SXGA. At a 60 Hz refresh rate, this corresponds to switching times ranging from 26 to 14 μs. In order to write the proper data voltage, Vd, which ranges typically from 2V to 12V (a +5 to −5 V range about the common voltage of approximately 7V, on alternating frames). The gate voltage, Vg, of the switching TFT is typically taken from an OFF level of about −5 V to an ON level on the order of +25 V. In this scenario, the switching TFT is always operating in the linear regime with Vg−Vth>Vd when the pixel is charging, going through saturation only briefly when the switching gate pulse is turned on or off while Vd is constant, where Vth is the TFT threshold voltage
In an AMOLED display, the luminance level is not a function of the final voltage applied to the LC cell, but rather is a function of the current level supplied by a drive TFT (see FIG. 1B). The switch TFT operates in the same fashion as the single TFT in the AMLCD unit cell. However, the data voltage is written onto a storage capacitor attached to the gate of the current drive transistor, and it is the threshold stability of this current drive TFT which must remain stable over a long period of operation (i.e., a good fraction of the frame time) for the AMOLED display to be commercially useful.
The belief in this technology area has always been that amorphous silicon TFTs do not have the performance needed for integration into the matrix addressed pixel to drive OLEDs (J. Kanicki et al, SID 20th IDRC Proceedings, September 25-28, Palm Beach, Fla., pp 354-358) and that all prototypes and products to date reflect this belief by using poly-silicon TFT technology.
The present inventors have developed the following unique drive schemes tailored explicitly to combat threshold shift, thus making the use of a-Si technology practical for AMOLED. Providing for amorphous silicon TFTs to meet the AMOLED requirements, such as that provided by the present invention, the less expensive amorphous silicon (a-Si) TFT technology compared to the more costly poly-SI TFT technology would provide substantially lower manufacturing cost.
The present invention also provides many additional advantages which shall become apparent as described below.